The present invention relates to planar optical waveguides and in particular, to a method of fabricating a silica-based planar optical waveguide.
The formation of optical waveguides is well known. Turning initially to FIG. 1, there is illustrated a sectional view of a planar channel waveguide structure 1 fabricated in accordance with prior art methods. The waveguide structure 1 comprises a silicon substrate 2 on which is deposited a silica buffer layer 3 followed by a silica-based core 4 and a subsequent silica cladding layer 5. A number of problems exist with prior art methods of fabricating this type of waveguide. In particular, prior art methods of depositing the various layers 3-5 on the substrate 2 can lead to the formation of thermal stresses due to differences between the thermal expansion coefficients of the substrate and the layers, which can in turn cause undesirable birefringence in one or more of the layers 3-5. If one or more of the layers 3-5 are sufficiently birefringent that a refractive index of an optical mode being guided in the waveguide structure 1 is affected by the polarisation of the mode, the waveguide is referred as being xe2x80x9cpolarisation sensitivexe2x80x9d. It is generally undesirable to produce waveguides which are polarisation sensitive because optical signals of different polarisations will not be processed identically. For example, a waveguide containing a Bragg grating which is polarisation sensitive can have a Bragg wavelength xcexB for TE-polarised light which is different to the Bragg wavelength of TM-polarised light.
Prior art methods of preventing waveguide birefringence include modifying the composition of the cladding layer 5 so as to match the coefficient of thermal expansion of the silicon substrate 2. This can be done through co-doping the silica cladding layer 5 with phosphorous and boron. However, these co-dopants often make the cladding layer susceptible to moisture penetration.
Other prior art methods of preventing waveguide birefringence include using: stress-relieving grooves in the silica cladding; polarisation-rotating polymer plates; or UV trimming. Each of these techniques has the disadvantage of complicating the fabrication process.
In accordance with a first aspect of the present invention, there is provided a method of manufacturing a waveguide structure which is substantially polarisation-insensitive, the method comprising:
(a) depositing a buffer layer on a substrate;
(b) depositing a core layer on the buffer layer and etching the core layer so as to form a waveguide core; and
(c) depositing a silica-based cladding layer over the core by means of plasma-enhanced chemical vapour deposition (PECVD) in the absence of nitrogen or nitrogen-containing gases so as to complete the waveguide structure, wherein the cladding layer is deposited in a manner which substantially prevents polarisation sensitivity in the waveguide structure.
Preferably, the step of depositing the cladding layer does not include an annealing step. The cladding layer may be deposited in a manner which makes it unnecessary to subsequently anneal or reflow the cladding layer. For example, post-deposition annealing or reflowing is normally used in prior art fabrication methods to reduce optical losses and to fill in gaps in the cladding layer. In the present invention, the cladding layer is preferably deposited with an optical transparency and surface coverage sufficient to make annealing unnecessary. Such an embodiment is particularly advantageous since annealing and reflowing can introduce polarisation sensitivity such a sensitivity was not previously present.
Preferably, the PECVD is carried out using deposition conditions selected such that any stresses induced by the cladding layer are distributed in a manner which substantially prevents polarisation sensitivity in the waveguide. The PECVD may be carried out using deposition conditions selected to induce an intrinsic stress in the waveguide structure which at least partially negates any thermal stress in the waveguide so as to substantially prevent polarisation sensitivity in the waveguide. Thermal stress will be understood to mean stress which arises as a result of a difference between thermal expansion coefficients of the cladding layer and substrate. Intrinsic stress will be understood to mean stress in the cladding layer which is not attributable to thermal stress.
Alternatively or in addition, the PECVD may be carried out using deposition conditions selected to induce a degree of stress in the core sufficient to substantially compensate for any form birefringence resulting from a geometry of the core.
The selected PECVD deposition conditions may provide a degree of ion bombardment and/or a deposition rate required to form the cladding layer in a manner which substantially prevents polarisation sensitivity in the waveguide structure. The required degree of ion bombardment may be provided by controlling one or more of the following parameters:
a frequency of RF power applied across electrodes used in the PECVD;
a level of RF power applied across the electrodes; and
a processing pressure during the PECVD.
The PECVD may be carried out using two RF power sources to input RF power into electrodes used in the PECVD, wherein the two sources operate at a lower frequency and an upper frequency respectively. In this case, the required degree of ion bombardment may be provided by controlling one or more of the following parameters:
a level of RF power applied across the electrodes by the lower frequency power supply;
a level of RF power applied across the electrodes by the upper frequency power supply;
an operating frequency of the upper and/or lower frequency power supplies; and
a processing pressure used during the PECVD deposition.
The required deposition rate may be controlled by controlling one or more of the following parameters:
a flow rate of a vapour-phase precursor used in the PECVD;
a process pressure during the PECVD; and
a substrate temperature during the deposition.
In one embodiment, cladding-layer-induced stresses are controlled by simultaneously or individually controlling ion bombardment, deposition rate, or substrate temperature during the cladding layer deposition.
A liquid source material may be used to form the cladding layer. For example, the liquid source material may comprise tetraethoxysilane (TEOS). The method may further include introducing refractive-index-modifying dopants into the cladding layer to compensate for any refractive index differences between the cladding layer and the buffer layer. Examples of refractive-index-modifying dopants include refractive-index-decreasing dopants such as fluorine and boron, and refractive-index-increasing dopants such as germanium and phosphorus. Examples of source materials for refractive-index-modifying dopants include tetra-methyl germanium (TMG), tri-ethyl phosphate (TEPO) and tri-ethyl borate (TEB).
The step of depositing the cladding layer may comprise depositing two or more sub-layers in which at least one sub-layer is deposited in a manner which substantially prevents polarisation sensitivity in the waveguide structure. This embodiment has the advantage that the remaining sub-layers can optionally be deposited at a higher deposition rate than the at least one sub-layer so as to optimise fabrication time. The method may further include depositing the plurality of sub-layers such that at least one sub-layer is under a degree of tensile stress and at least one sub-layer is under a degree of compressive stress, wherein the sub-layers are arranged such that the net stress in the cladding layer substantially prevents polarisation sensitivity. In one embodiment, the tensile and compressive stresses substantially cancel each other out such that the net stress in the cladding layer is substantially zero. The stress in each sub-layer may be made more tensile (less compressive) by increasing the deposition rate. Also, the stress in each sub-layer may be made more tensile stress by decreasing the average ion bombardment energy. Thus, in general, the stress in each sub-layer can be made more tensile (less compressive) by increasing the ratio of deposition rate to average ion bombardment energy.
In accordance with a second aspect of the present invention, there is provided a method of manufacturing a waveguide structure which is substantially polarisation-insensitive, the method comprising:
(a) depositing a buffer layer on a substrate;
(b) depositing a core layer on the buffer layer and etching the core layer so as to form a waveguide core; and
(c) depositing a silica-based cladding layer over the core, the cladding layer being formed by:
(i) depositing an initial silica-based layer over the core by means of plasma-enhanced chemical vapour deposition (PECVD) in the absence of nitrogen or nitrogen-containing gases; and
(ii) subsequently annealing the initial silica-based layer so as to form the cladding layer, wherein the annealing and the PECVD are carried in a manner which substantially prevents polarisation sensitivity in the waveguide structure, and the annealing is carried out at a temperature which is higher than a deposition temperature during the PECVD but sufficiently low to substantially avoid reflowing of the initial silica-based layer
Preferably, the PECVD and annealing are carried out under conditions selected such that any stresses induced by the cladding layer are distributed in a manner which substantially prevents polarisation sensitivity in the waveguide. This aspect of the invention has applications where other fabrication considerations make it necessary to subject the waveguide structure to an annealing process For example, if the waveguide structure is monolithically integrated on a common substrate with another optical component which requires annealing, it can be necessary to expose the waveguide structure to the same annealing conditions. Prior art waveguide structures have the disadvantage that they can be rendered polarisation sensitive as a result of annealing-induced thermal stresses. In the present invention, the cladding layer is deposited such that subsequent annealing substantially eliminates any polarisation sensitivity. In one embodiment, the PECVD and annealing are carried out using conditions selected such that the initial silica-based layer undergoes an increase in density during the annealing, wherein the increase in density is sufficient to at least partly counteract any thermal and/or intrinsic stress in the initial silica-based layer. The increase in density may cause a slight film shrinkage which at least partly relieves compressive stress in the waveguide structure. Such an embodiment has the advantage that the minimum annealing temperature required to alleviate stress in a low-density silica-based layer by densification can be lower than the temperature required to cause the layer to reflow. The stress counteracted during the annealing my comprise compressive stress. Preferably, the stress relief which occurs during the annealing is of a sufficient magnitude to substantially prevent polarisation sensitivity in the waveguide structure.
In accordance with a third aspect of the present invention, there is provided a waveguide structure fabricated in accordance with any one of the above-described methods.
It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.
For the purposes of this specification it is to be clearly understood that the word xe2x80x9ccomprisingxe2x80x9d means xe2x80x9cincluding but not limited toxe2x80x9d, and that the word xe2x80x9ccomprisesxe2x80x9d has a corresponding meaning.